Implement file decoration API

Created by: tjkandala

The File Decoration API now belongs to the new app namespace (scrapped tree).

Components in the web app request decorations for one level in the tree at a time, expect for single child directories, in which case the web app asks for decorations for that node and all of its children. Since it's possible that multiple components request decorations for the same entries, we memoize the observable of decorations (cache key based on parent node's uri).